1. Field of the Invention
The present invention relates to a magneto-resistance effect head used in a magnetic recording system and a magneto-resistance effect element.
2. Related Art
In some kinds of ferromagnetic substance, a phenomenon where an electric resistance varies according to intensity of an external magnetic field has been known, and it is called “magneto-resistance effect”. This effect can be used for detecting an external magnetic field, and such a magnetic field detecting element or device is called “a magneto-resistance effect element” (hereinafter, referred to as an “MR element”).
Such an MR element is used for reading of information or data stored in a magnetic recording medium in a magnetic recording apparatus such as a hard disc, a magnetic tape or the like in an industrial field, and such a magnetic head is called “an MR head”.
In recent years, in a magnetic recording apparatus utilizing these MR elements, particularly, in a hard disc, a densification of a magnetic recording density is forwarded, a size of one bit is made small, and an amount of leakage magnetic flux from the bit is reduced increasingly. For this reason, it is made essential for reading of information or data written in a magnetic medium to manufacture an MR element with a high sensitivity and a high S/N ratio which can achieve a large resistance change rate even in a low magnetic field, which constitutes an important fundamental technique for improving a recording density.
Here, the high sensitivity means that a resistance change amount (Ω) per unit magnetic field (Oe) is large. The larger resistance amount and the more excellent soft magnetic characteristic an MR element has, the higher sensitivity the MR element has. Further, in order to realize a high S/N ratio, it becomes important to reduce thermal noises as much as possible. For this reason, it is not preferable that an element resistance itself becomes excessively large. When the element is used as a sensor for reading a hard disc, it is desired in order to realize an excellent S/N ratio that the element resistance is a value in a range of 10Ω to 200Ω.
Under such a background, it currently becomes common to use an MR element having a spin valve film which can obtain a large MR ratio as the MR element used in an MR head for a hard disc. The spin valve film is a film in which one ferromagnetic layer (also called “a magnetization pinned layer”) of two ferromagnetic layers which are put in a magnetically non-coupling state via an intermediate layer of non-magnetic material has magnetization fixed by exchange bias using an anti-ferromagnetic layer and the other ferromagnetic layer (called “a magnetization free layer”) can be put in a state where it can be easily magnetization-rotated by an external magnetic field(signal magnetic field or the like), so that a relative angle between the magnetization directions of the two ferromagnetic layers is varied by rotating only the magnetization of the magnetization free layer by an external magnetic field, thereby obtaining a large magneto-resistance effect (refer to, for example, Phys. Rev. B45, 806 (1992), and J. Appl. Phys. 69, 4774 (1991)). Since the spin valve film can rotate the magnetization in a low magnetic field, a high sensitization is possible and is suitable for an MR element for an MR head.
At present, a system which causes a sense current to flow from one electrode to the other electrode in parallel with a film face of a magneto-resistance effect film to such a spin valve film to measure a resistance in a direction parallel to the film face is commonly used. This system is generally called “a CIP (Current In Plane) system.
In this CIP system, the MR ratio can reach a value in a range of about 10% to 20%. In a current MR head of a shield type which is used commonly, since the spin valve film is used with a shape approximating to a regular square, the resistance of the MR element becomes approximately equal to a sheet resistance of the spin valve film. For this reason, in the spin valve film in the CIP system, it is possible to achieve an excellent S/N characteristic by setting the sheet resistance to 10Ω to 30Ω. This can be realized relatively easily by making the film thickness of the whole spin valve film thinner. In view of these merits or advantages, the spin valve film of the CIP system is commonly used as the MR element for a MR head at present.
However, for the purpose of realizing information reproduction with such a high recording density as exceeding 500 Gbpsi (Gigabit per square inch), it is prospected that the MR ratio must reach a value exceeding 50%. On the other hand, it is difficult to achieve a value exceeding 20% as the MR ratio in a conventional spin valve film. For this reason, it is a large technical problem to be solved for further improving a recording density to achieve how to make the MR ratio large.
As means for solving such a technical problem, a method which causes a sense current to flow from one electrode to the other electrode perpendicularly to a film face of a magneto-resistance effect film to measure a resistance, in the direction perpendicular to the film face, of the magneto-resistance effect film is known. This method is generally called “CPP (Current Perpendicular to Plane) system. MR elements used in the CPP system are largely classified to three kinds. The first one of the kinds has a structure that non-magnetic metal is used for an intermediate layer, which is generally called “a CPP-spin valve film”. The second one has a structure that insulator is used for an intermediate layer, which is generally called “a magnetic tunnel MR film”. The third one has a structure where an intermediate layer is constituted by contact points between magnetic substances, which is generally called “a point contact MR film”.
In the CPP system, an operation principle is based upon that a large MR ratio is obtained by utilizing the fact that a conductance between two magnetic layers joined via (1) non-magnetic metal, (2) insulator or (3) contact points is changed in response to a change of a relative angle in magnetization between the two magnetic layers. That is, it is made possible to cause a current to flow in a direction perpendicular to a magnetic layer/an intermediate layer/a magnetic layer to cause most of the current to cross the same, thereby utilizing an excellent interface effect. For this reason, it is known that a large MR ratio exceeding 50% can be obtained in the above three elements in principle. Therefore, it is essential to use a magneto-resistance effect element of the so-called CPP system in an MR head corresponding to a high magnetic recording density exceeding 500 Gbpsi.
Among them, the CPP-spin valve film having a possibility that a low resistance and a large MR ratio are compatible with each other is attracting as an MR element for a head. Incidentally, it is necessary to maintain the thickness of the magnetic layer in a range of about 5 nm or less in this structure. When such an extremely thin magnetic layer is used, conductive electrons which have not been polarized in spin and have been injected from an electrode into a magnetic layer pass through the magnetic layer before they have been polarized in spin. As a result, there exists such a problem that it becomes difficult to obtain a sufficient MR ratio due to lack in spin polarization of conductive electrons.
Further, in the CPP-spin valve film, the resistance of an anti-ferromagnetic layer which contributes to only increase in element resistance but does not contribute to a spin-dependent resistance causing MR change is connected to an entire current path in series, so that the resistance causes increase in element resistance and decrease in MR ratio when judgement is made as the whole element.
In order to use the CPP-spin valve film as the MR element for a head, it is necessary to solve the above two problems.
On the other hand, as the structure for the MR head, a shield type MR head where an MR element is provided so as to be opposed to a medium between shields enters a mainstream. In such a shield type MR head, however, there are some problems, and it is therefore considered that it is difficult to use the head at a recording density equal to 500 Gbpsi or more.
One of the problems is a problem about a gap distance or interval, where a line recording density is defined by a gap distance between shields in the shield type MR head, but it is necessary to set the gap distance to be extremely fine such as 30 nm or less in case of a high density exceeding 400 Gbpsi. Therefore, it becomes much difficult to sandwich or insert a MR element in such a fine gap. This is because the thickness of only the MR element has a value close to 20 nm.
The second of the problems is a problem of a depth process, where a depth of an MR element is determined by polishing in the shield type MR head at the final stage. It is necessary to control the depth of the MR element with accuracy of 10 nm or less after polishing run-in in a case of high density exceeding 500 Gbpsi. However, it is not easy to achieve such accuracy by machining.
As described above, in a high recording density exceeding 500 Gbpsi, it is essential to use a CPP-MR element which can obtain a large MR ratio by conducting a current in a direction perpendicular to a film face of a magneto-resistance effect film. In particular, it is expected that the CPP-spin valve element from which a high MR ratio can be expected with a low resistance is used while avoiding the problems therein.
Moreover, a novel MR head structure provided with a fine resolution replaced for the shield type MR head is required in order to meet the high density recording exceeding 500 Gbpsi.
On the other hand, regarding the perpendicular magnetic recording, there is such a problem as described below.
In generally, a record reproducing system in a current magnetic recording apparatus employs a longitudinal recording system. In the longitudinal recording system, there will occur such a problem that demagnetization field becomes larger according to increase in recording density, which results in lowering of reproduction output and failure in stable recording. A perpendicular recording system has been proposed as means for solving such a problem. In this perpendicular recording system, magnetization and recording are conducted in a direction perpendicular to a recording medium face, where, even if a recording density is increased as compared with the longitudinal recording, influence of demagnetization field is reduced and lowering of reproducing output or the like is suppressed. Therefore, the perpendicular recording system has been regarded as importance.
The longitudinal recording and the perpendicular recording are different in reproducing wave shape which has not been subjected to a signal processing yet. When a signal processing for reproduction is carried out using the perpendicular recording, it is necessary to modify the reproduction signal processing technique of the longitudinal recording which has been cultivated by this time. The modification methods will be classified roughly to two methods, one method which includes conducting a differential processing on a raw reproduction waveform to form a single peak waveform like the case of the longitudinal recording and thereafter using the signal processing technique for the longitudinal recording, and the other method which includes establishing a technique for the perpendicular recording totally. The former method is easier than the latter method, but such a possibility becomes high that the former is inferior in error rate to the latter.
Even if a signal processing is conducted by either method in the perpendicular recording system, it is necessary to sharpen a raw reproduction waveform in order to achieve a high recording density. As an index indicating sharpness of a reproducing waveform, PW 50 showing a half width is used in case of a single-humped wave such as a differentiated wave in the longitudinal recording system or the perpendicular recording system and T50 (a time period from 25% output to 75% output) is used in case of a monotone wave like the perpendicular recording. This value is controlled to about 2.0 to 3.0×bit length in the case of PW50, and it is controlled to 1.4 to 2.0×bit length or the like in the case of T50 so that a desired error rate is obtained. The value largely depends on a reproduction gap length. Therefore, in order to make line density large, that is, make a bit length small, it is necessary to make the reproducing gap length small.
However, making the reproduction gap small is approaching to a limitation. A total film thickness of a magneto-resistance effect film used in a product level has at least 20 nm or so at present, and when the thickness of an insulating layer is added to the total film thickness, the limitation of the reproduction gap will be 50 nm at present. Even if it is assumed that a withstand voltage of the insulating layer becomes large or a CPP element or the like is used, the limitation of the reproduction gap is considered to be 30 nm, so that an application limitation of a current shield type will be estimated to be 400 Gbpsi or so. In order to achieve a further high recording density exceeding this limitation, if the reproducing head of the current shield type is used, it is considered that a breakthrough of a signal processing system or the like will be required.
On the other hand, a reproducing head of a yoke type has been conventionally considered as a reproducing head where no shield is used. The reproducing head of the yoke type is constituted such that magnetic flux from a medium is prevented from flowing into a magneto-resistance effect element directly by spacing ABS (air bearing surface), or a medium opposing face and the magneto-resistance effect element from each other, a soft magnetic substance (in many cases, a pair of magnetic substance) called “a yoke” is exposed to the ABS, and the ABS is connected to the magneto-resistance effect element so that magnetic flux is led to the magneto-resistance effect element. Thereby, it is considered that sensitivity is concentrated to the vicinity of an exposed portion of the yokes at the ABS so that a reading resolution is improved and differentiation of the yokes provided in the paired manner serves so that the resolution is increased. Further, since the magneto-resistance effect film portion does not have sensitivity directly, a magneto-resistance effect element section can be made large, which results in merit in manufacture. Furthermore, the differentiation in the yokes influences a reproduction waveform like the differentiating circuit in a reproduction signal processing. In particular, when the reproducing head of the yoke type is used in a perpendicular magnetic recording, it becomes a single peak wave in the same manner that the shield type reproducing head is used in the longitudinal recording system. For this reason, it is considered that the same reproduction signal processing system as the conventional longitudinal recording system can be used without using a differentiating circuit.
However, as the result of eager studying, it has been found that the reproducing head of the yoke type actually has some sensitivity even at its portion spaced from the ABS, and in particular, when the reproducing head is used in the perpendicular recording system, there is a defect that a resolution having such a degree that a predetermined error rate can be obtained can not be obtained. In case that the yoke type head is used in the perpendicular recording system, magnetic flux sensing by the yoke when there is a gap in a magnetization transition point becomes maximum and a reproduction waveform forms a single-humped wave. When the gap deviates from the magnetization transition point, an output is principally expected to become zero. However, it has been found from the result of research that because magnetic flux from the medium reaches an upper portion of the yoke spaced from the ABS face, a spatial resolution is remarkably lowered.